As known in the art, high selectivity catalysts (HSCs) for the epoxidation of ethylene refer to those catalysts that possess selectivity values higher than high activity catalysts (HACs) used for the same purpose. Both types of catalysts include silver as the active catalytic component on a refractory support (i.e., carrier). Typically, one or more promoters are included in the catalyst to improve or adjust properties of the catalyst, such as selectivity.
Generally, but not necessarily always, HSCs achieve the higher selectivity (typically 87 mole % or above) by incorporation of rhenium, and or an oxyanion of tungsten, molybdenum, or chromium as promoters. Typically, one or more additional promoters selected from alkali metals (e.g., lithium, potassium, and/or cesium), alkaline earth metals, transition metals (e.g., tungsten compounds), and main group metals (e.g., sulfur and/or halide compounds) are also included.
There are also ethylene epoxidation catalysts that may not possess the selectivity values typically associated with HSCs, though the selectivity values are improved over HACs. These types of catalysts can also be considered within the class of HSCs, or alternatively, such catalysts can be considered to belong to a separate class, e.g., “medium selectivity catalysts” or “MSCs.” These types of catalysts typically exhibit selectivities of at least 83 mole % and up to 87 mole %.
It is well known that with extended use of a catalyst, the catalyst will show signs of ageing (i.e., degraded performance) to a point until use of the catalyst is no longer practical. For obvious reasons, there is a continuous effort to extend the useful lifetime (i.e., “longevity” or “usable life”) of the catalyst. The useful lifetime of the catalyst is directly dependent on the stability of the catalyst. As used herein, the “useful lifetime” is the time period for which a catalyst can be used until one or more of its functional parameters, such as selectivity or activity, degrade to such a level that use of the catalyst becomes impractical.
Stability of the catalyst has largely been attributed, in part, to various characteristics of the carrier. Some characteristics of the carrier that have undergone much research include carrier formulation, surface area, porosity, particle morphology, and pore volume distribution, among others.
The most widely used formulation for the carriers of ethylene epoxidation catalysts are those based on alumina, typically α-alumina. Much research has been directed to investigating the effect of the alumina composition for improving stability and other properties of the catalyst.
For example, the presence of sodium (Na) in an α-alumina carrier plays an important role in the ageing of an ethylene oxide catalyst. This fact was recognized for some time and several publications show evidence that confirmed the degrading effect of Na present on the surface of the carrier. For instance, ISS analysis showed the increase of Na and chloride (Cl) on the surface as the catalyst ages. XPS data showed that the binding energy of both surface Na and Cl corresponds to the formation of NaCl on the surface of the aged catalyst, Cl is adsorbed from the gas feed. See, for example, the publications to G. Hoflund and D. Minahan entitled “Study of Cs-promoted α-alumina-supported silver, ethylene epoxidation catalysts” Journal of Catalysis, 162, 1996, 48 and “Ion-beam characterization of alumina-supported silver catalysts used for ethylene epoxidation” Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 118, Issues 1-4, 1996, 517. It was suggested in the aforementioned publications that Na migrates from the binder to the surface is accelerated by the driving potential provided by the surface chloride.
There are many publications which describe α-alumina carrier treatment processes that are aimed at improving the catalytic performance, e.g., stability of the resultant catalyst that is formed on the treated carrier. The processes generally wash the carrier prior to impregnating the carrier with silver and other promoters. The prior art treatment processes are limited in scope in carrier treatments that deal generally with removing Na from the surface of the carrier. For instance, both U.S. Pat. Nos. 2,901,441 and 3,957,690 disclose a procedure for washing a carrier of a silver catalyst. In these publications, the α-alumina carrier is washed by heating in a hot aqueous solution of organic acid and then rinsed with water. U.S. Pat. Nos. 5,102,848 and 5,504,053 disclose washing of an α-alumina carrier using hot water. Similarly, U.S. Pat. No. 6,103,916 discloses washing of an α-alumina carrier for an ethylene oxidation catalyst. In the '916 publication, washing is aimed to remove “leachable Na”. To test the carrier for its leachable Na, the washed carrier is boiled in water and the resistivity of the drained water is more than 10,000Ω. Also, U.S. Pat. Nos. 6,368,998, 6,579,825, 6,656,874, and 7,439,375 disclose a process of lowering a concentration of ionizable species present on the surface of the carrier. The ionizable species, especially silicates, were extracted by boiling in deionized water. The process was repeated 3 times for 15 minutes, each.
DE2933950 shows experimental evidence that soluble alkali metal silicates are responsible for degrading the catalytic performance. The EP '950 publication discloses a process of washing the carrier with a hot NaOH solution to remove these salts and improve the catalytic performance. Also, U.S. Pat. No. 6,846,774 discloses washing the alumina carrier, of an ethylene oxide catalyst, with a hot aqueous basic solution and maintaining the pH of the solution above 8.
Despite the above washings which remove Na from the surface of the carrier, the above carrier treatments do not purport to remove Na from the subsurface of the carrier. As such, and during the preparation of the catalysts and during its lifetime this subsurface Na can migrate to the surface of the finished catalyst. For example, the silver impregnated solution contains high levels of soluble Ag, as a salt or a complex, along with additives, which include different levels of alkali metal promoters. These alkali metals are capable of penetrating to the subsurface of the carrier and can replace the Na within the subsurface of the carrier. The replaced Na that was previously located within the subsurface of the carrier migrates to the surface of the finished catalyst. This ion exchange process will lead to an impregnating solution, inside the pores, that also contains soluble Na. The alkali metals exchange continues even during the drying and calcining phases of the catalyst's preparation. The result is that, even after washing the carrier, the catalyst will have an undesirably higher level of Na on its surface, which can significantly degrade the performance of the catalyst. The process of alkali metals exchange will also continue during the catalyst's life. This is because the presence of alkali metals, promoters, on the surface of the carrier acts as a driving force to the flux of Na ions from the carrier's subsurface to the surface of the catalyst.
In view of the above, there is a need for providing a silver-based ethylene oxide catalyst which includes a reduced content of sodium on the surface of the finished catalyst.